Analysis of the textural properties of litchi fruit using puncture and TPA methods

【Objective】This study aims to explore the testing conditions and methods suitable for the texture characteristics of litchi fruit through whole fruit puncture test and Texture Profile Analysis (TPA). Puncture test refers to the use of a millimeter diameter probe to puncture a sample, which can determine the hardness and brittleness of the sample, and it is also commonly used for peel analysis. TPA testing simulates the chewing motion of the human oral cavity and compresses the sample twice. This process can determine the texture parameters such as hardness, adhesion, elasticity, cohesion, and chewiness of the sample. Meanwhile, through variance analysis, correlation analysis, principal component analysis, and cluster analysis of the texture parameters of different lychee varieties, a preliminary quantitative evaluation method for lychee fruit quality was established.【Methods】This study used mature Shuidong fruits as experimental materials. Firstly, physiological indicator data of fresh fruits were collected, including fruit weight, fruit volume, fruit size, etc. Then, using puncture and TPA methods, referring to relevant literature, single factor tests were conducted on indicators such as deformation, testing speed, puncture depth, and trigger force. To clarify the optimal conditions for determining the texture of lychee fruit, we set multiple gradients for each detection factor in this experiment. Prior to the start of the formal experiment, pre- experiments were conducted on the test samples and basic testing conditions. And three fruit texture analysis and testing modes have been established: whole fruit puncture method, whole fruit TPA method, and whole fruit peeling TPA method. During testing, the suture line of the fruit is parallel to the storage platform, with the highest point on the sunny side as the testing point. Five fruits were measured for each treatment. In addition, this study used 8 lychee cultivars including Shuidong, Feizixiao, Chiye, Renshanli, Zaomi, Heiye, Leiling Xiaolingxiang, and Ziniangxi as experimental materials to analyze and evaluate the texture of lychees and test the adaptability of the optimal conditions.【Results】(1) The results of the single factor experiment showed that the optimal parameters for detecting lychee fruit peel by puncture method were puncture depth of 4 mm, testing speed of 0.5 mm·s -1 , and triggering force of 0.38 N; The optimal testing level for TPA testing of lychee whole fruit (with and without skin) is a deformation of 20%, a testing rate of 0.5 mm·s -1 , and a triggering force of 0.38 N. (2) The analysis of fruit texture parameters of different lychee varieties showed that there were differences in texture indicators between each variety in the puncture test and TPA test. Among them, in the puncture test, the variation amplitude of peel strength was the smallest, with a coefficient of variation of 14.35%. The peel strength of Feizixiao was the highest, significantly higher than the other seven varieties; In the TPA whole fruit with skin analysis, the recovery of Feizixiao was significantly higher than that of other varieties except for Chiye; In the whole fruit peeling TPA experiment, the chewiness of the flesh of Renshanli and Zaomi was significantly higher than that of other varieties (P＜0.05), while there was no significant difference in the chewiness of the flesh of Heiye, Shuidong, Leiling Lilac, and Ziniangxi. (3) The correlation analysis of texture indicators of different lychee varieties showed that the skin hardness was very significantly and positively correlated with the maximum displacement of the peel and the penetration of the fruit peel (P＜0.01), the hardness of the fruit flesh was highly significantly and positively correlated with chewiness (P＜0.01), and highly significantly and negatively correlated with cohesiveness and resilience (P＜0.01). Elasticity and chewiness were highly significantly and positively correlated (P＜0.01). These results indicated that important indicators in the puncture test included peel hardness, peel maximum displacement, and puncture work, while important indicators in the TPA test included fruit hardness, cohesiveness, resilience, chewiness, and elasticity. (4) Using principal component analysis, 6 factors were extracted from 28 indicators (phenotype indicators and texture analyzer parameter indicators), with a cumulative variance contribution rate of 87.90%, representing the vast majority of texture characteristics, which can basically analyze the main reasons for the diversity between different lychee germplasm resources. The first and second principal components can be used as important indicators for variety identification. Fifteen indicators with load values greater than 0.8 were extracted for cluster analysis, and the texture indicators were divided into three categories. Eight lychee cultivars were also divided into three categories, indicating that the selected important indicators can be used to distinguish the texture characteristics of different lychee fruits.【Conclusion】 Compared with TPA test, whole fruit puncture test is more suitable for analyzing the texture of fruit skin. In TPA, the compression of whole fruit with skin can also reflect the texture characteristics of fruit flesh to a certain extent. This study preliminarily determined the standardized parameters for lychee puncture test and TPA test and screened out the core indicator group that can be used to evaluate the texture characteristics of lychee fruit. The combination of puncture method and TPA method can effectively distinguish the texture differences between different germplasm resources, enriching the content of ly-chee texture evaluation.